The present invention relates to a system for computing a road surface frictional coefficient according to output values of a vehicle body lateral acceleration and a vehicle body fore-and-aft acceleration when tire are slipping.
According to a conventional system for computing a road surface frictional coefficient, output values of a vehicle body lateral acceleration and a vehicle body fore-and-aft acceleration when tire are slipping are detected, and the road surface frictional coefficient is computed from these detected values.
However, according to such a conventional method, the road surface frictional coefficient cannot be updated according to the change in the condition of the road surface unless the tire slip ratio gets greater than a certain level. For instance, when the tire grip force is relative small on a low frictional coefficient road such as a snow-covered road and frozen road, the tire slip ratio is so small that the road surface frictional coefficient tends to be estimated higher than actually is.
In view of such problems of the prior art, a primary object of the present invention is to provide a system for computing a road surface frictional coefficient which is free from such problems of the prior art.
A second object of the present invention is to provide a system for computing a road surface frictional coefficient which provides an accurate value under all conditions.
According to the present invention, these and other objects of the present invention can be accomplished by providing a system for computing a road surface frictional coefficient for controlling a motion of a vehicle, comprising: a vehicle body fore-and-aft acceleration sensor for detecting a vehicle body fore-and-aft acceleration; a vehicle body lateral acceleration sensor for detecting a vehicle body lateral acceleration; a wheel speed sensor for detecting a road wheel speed; a vehicle body speed estimating unit for computing a vehicle body speed according to the wheel speed; a tire slip ratio computing unit for computing a tire slip ratio according to the vehicle body speed and wheel speed; a tire model defining unit for defining a dynamic tire model; a tire fore-and-aft force computing unit for computing tire fore-and-aft force according to the tire slip ratio and tire model; an estimated vehicle body fore-and-aft acceleration computing unit for estimating the vehicle body fore-and-aft acceleration according to the tire fore-and-aft force; a tire lateral force computing unit for computing a tire lateral force according to the tire slip ratio and tire model; an estimated vehicle body lateral acceleration computing unit for estimating the vehicle body lateral acceleration according to the tire lateral force; and a road surface frictional coefficient computing unit for computing a road surface frictional coefficient according to at least one of a result of comparison between the detected vehicle body fore-and-aft acceleration and estimated vehicle body fore-and-aft acceleration, and a result of comparison between the detected vehicle body lateral acceleration and estimated vehicle body lateral acceleration.
Thus, the road surface frictional coefficient is estimated according to the deviation of the estimated vehicle body fore-and-aft/lateral acceleration from the actually detected vehicle body fore-and-aft/lateral acceleration, instead of estimating it directly from the tire slip ratio so that the road surface frictional coefficient is prevented from being estimated substantially higher than it actually is. In particular, by judging a sudden change in the road surface frictional coefficient only when a change in the road surface frictional coefficient, in particular an increase in the frictional coefficient, has persisted for more than a prescribed time period, an error in the estimation of the road surface frictional coefficient can be minimized, and a favorable vehicle motion control based on the estimated road surface frictional coefficient can be ensured under all conditions.